Informal Car meetings

I'd most likely say ( in my cars' case) "Neither is the gas that is in it."
"And since you are the one who brought it to my attention, the motor oil, transmission and rear end lube, radiator coolant and windshield wipers are not original either.
Anything else you would like to add ?? "

They usually go away mumbling to themselves and pointing back at me to their buddies after they are about 30 feet away.

Gotta love it.
DO NOT let them get to you.

Just keep your eye on them so one of them doesn't come back and run something sharp down the side. Saw it happen one time (not to me tho)